Dual Nature of Light


                During the 19th century, it has been an unanswered question that what is the nature of light? Either is light a wave or particle?

            Many scientists represented their ideas and theories to solve this problem. Their ideas were being rejected by one another. Some of them proposed their theories with experimental help. But some remained unsuccessful to give any evidence about their views for the nature of light. After all, we got the answer as a fruit of the struggle of all the scientists. We are describing briefly the contribution of some nominal scientists to clarify the nature of light.

Nature of Light

                It is an admitted fact that light has dual nature. It behaves as waves when travels such as in phenomena of interference, diffraction and polarization. It behaves as particles when collide with matter such as in phenomena of photoelectric effect (the emission of electrons from a metal surface when it is exposed to a light of suitable frequency) and Compton effect (the increase in the wavelength of electromagnetic radiations when they scatter from a target).

What is Optics

            When we study the behavior of light scientifically, it is called optics. It covers reflection, refraction, diffraction, interference and polarization of light. Any successful theory about nature of light must be able to explain all these and other optical phenomena.

Theories of Light

            The earlier scientific theories about nature of light were proposed around the end of 17th century.

Huygens’s Theory of Light

                Christian Huygens proposed a theory about nature of light in 1960 that light is a wave phenomenon.

Newton’s Theory of Light

            A rival theory to Huygens’s theory was offered by Sir Isaac Newton in 1740. Newton discovered the visible spectrum in 1666. Then he stated that light is composed of tiny particles or corpuscles emitted by luminous bodies. He was able to explain many optical phenomena by combining this corpuscular theory with his laws of mechanics.

            For more than 100 years, Newton’s corpuscular theory of light was favored over the wave theory of Huygens partly because of Newton’s great prestige and enough experimental evidence comparison between the two theories.

Young’s Double Slit Experiment

            Finally, some important experiments were done on diffraction and interference of light by Thomas Young and Fresnel in 1801. The result of these experiments could only be interpreted in terms of wave theory of light. Still, the phenomenon of polarization of light could only be explained by the wave theory. Thus wave theory became the dominant theory of the nature of light in the 19th century.

Maxwell’s Theory of Light

                The Huygens wave theory of light received additional support from electromagnetic theory of light by James Clerk Maxwell. He showed electric and magnetic fields propagate together in space and their speed is identical with the speed of light. It thus became clear that visible light is a form of electromagnetic radiation which constituting only a small part of the electromagnetic spectrum. Hertz discovered of radio waves in 1886. After that Maxwell’s theory was confirmed experimentally.

Modern Theory of Light

            The two general problems called ether hypothesis and explanation of photoelectric effect remained when electromagnetic theory of light was accepted.

Ether Hypothesis

                A hypothesis medium was suggested by the early scientist as the carrier of light waves just as air or water carries sound waves. It was assumed that ether had some very unusual properties, e.g. being mass-less but having high elasticity.

            A number of experiments such as Michelson and Morley experiment in 1887 were performed to give evidence of the ether but all failed to support the ether hypothesis. The ether was shown to be unnecessary to the electromagnetic theory when special theory of relativity was published in 1905 by Elbert Einstein.

Explanation of Photoelectric Effect

            The second main and serious was explanation of various phenomena such as photoelectric effect that involves interaction of light with matter. The solution to the problem was again proposed by Elbert Einstein in 1905 when he explained photoelectric effect.

Photon Theory of Light

                Elbert Einstein extended the quantum theory of thermal radiation proposed by Max Planck in 1900 and suggested that light and other form of electromagnetic radiations travel as tiny bundles of energy called quanta or photons. The energy of every photon is related directly to its frequency. Higher frequency refers to higher energy and lower frequency refers to lower energy.

            It became apparent that light and other forms of electromagnetic radiations are emitted and absorbed during transition process (transition process means when an electron jumps from high energy level to low energy level or from low energy level to high energy level) when quantum theory of atomic and molecular structure was established by Bohr and other scientists.

Dual nature of Light

                Louis de Broglie introduced wave-particle duality concept in 1924. All aspects of the behavior of light were successfully explained with the help of quantum theory of light.

            Today we admit that live has a dual nature. It behaves as waves when travels such as in phenomena of interference, diffraction and polarization. It behaves as particles when collide with matter such as in phenomena of photoelectric effect and Compton effect.